In X-ray tubes of conventional constructions it has been common practice to dispose a substantially round exit window of the tube as close as possible to the focal spot on the target where X-radiation is generated. This enables the tube to emit through the window a conical beam which, because of its proximity to the focal spot, increases greatly in cross-sectional size as it passes in a direction away from the tube. This enables the tube to irradiate a relatively broad or expansive area, the eventual shape and size of which may be controlled by passing the beam through a suitable collimator which intercepts marginal portions of the beam so that the radiation will be controllably confined to a predetermined area.
In some tubes used for specific purposes it is desirable to produce a beam having a fan shape. This is achieved by providing the exit window with an elongated rectangular configuration. A tube having such a window is shown and described in U.S. Pat. application Ser. No. 719,026, which is owned by the assignee of the present invention. However, while the X-ray beam in such tubes is substantially fan-shaped, it still has some undesired divergence in the shorter direction of the window since the focal spot from which the X-rays are emitted is considerably smaller than the size of the window opening.
In computerized axial tomography, for example, it is highly desirable that an X-ray beam be emitted which produces at the desired image plane a radiation pattern which is long and narrow in shape, the narrow dimension being desired so that at any one time a single linear array or row of detectors may be irradiated while adjacent parallel rows on either side of the first row will not be contacted by the beam. It will be apparent that if much divergence in the X-ray beam exists, the adjacent rows of detectors may be undesirably irradiated.
It has also been found that scattered radiation from other internal parts of the tube sometimes escapes and contributes undesirably to the irradiation of the detector system.